They are the same bugs, or at least have the same root cause ([MC-99179] is a duplicate of this, and the behavior you're describing would also fall under [MC-99179]).
Also affects lava, flowing lava, the lower part of cobwebs, sideways end rods, chorus plant, stairs if the minecart falls onto the lower part, trapdoors, enchanting tables, end portal frames, end portals, daylight detectors, turtle eggs, conduits, flower pots, skulls and the lower part of bubble columns. But I don't think that should all be listed in the description, it already says "any non-full-height block, including [list]". Btw, I tested none of the blocks in the list, I only looked at the list of blocks in the wiki.
In the full flow of the conversation, did someone point out specific use cases where this is a "bad thing"? I've seen various use cases including https://youtu.be/wd8l6AsAgCM where this is used to solve specific problems to do with minecarts.
Behavioural note: it seems the minecart can't go through such a block anymore like it used to. It would snap through the slab (other affected blocks not tested) and get stuck, as opposed to continue on the rail. I'm unsure whether this is caused by another bug, as there are other new bugs with minecarts.
Confirmed in 13w02b.